1. Field of the Invention
The present invention relates to color combining optical systems used for image projection apparatuses such as projectors.
2. Description of Related Art
An image projection apparatus combining reflective liquid crystal display elements and polarization beam splitters is disclosed in Japanese Patent Application Laid-Open No 2001-154268.
As shown in FIG. 12, the image projection apparatus disclosed in this publication includes a white light source 1001, reflective liquid crystal display elements 1002R, 1002G and 1002B, and a projection optical system 1003, as well as a dichroic mirror 1004 arranged between the white light source 1001 and the reflective liquid crystal display elements 1002R, 1002G and 1002B. The image projection apparatus further includes a color separating system provided with polarization beam splitters 1005 and 1006 between the dichroic mirror 1004 and the reflective liquid crystal display elements 1002R, 1002G and 1002B, as well as a color combining system provided with the first and second and a third polarization beam splitter 1005, 1006 and 1007 between the reflective liquid crystal display elements 1002R, 1002G and 1002B and the projection optical system 1003.
Here, the color separating system separates the white light with the dichroic mirror 1004 into light of a first color (green) and light of a second and third color (red and blue). The light of the first color is incident on a first polarization beam splitter 1005. The light of the second and third colors is incident on a first color-selective wave plate 1008 provided between the dichroic mirror 1004 and the second polarization beam splitter 1006. The first color-selective wave plate 1008 can rotate the polarization direction of the light of a predetermined wavelength region by 90°. Thus it is possible to correlate the color components (R and B) with the polarization directions (P and S).
The first color-selective wave plate 1008 rotates the polarization direction of the B light by 90°, the B light is incident as P-polarized light and the R light is incident as S-polarized light on the second polarization beam splitter 1006, and the light of the second color (R) is separated from the light of the third color (B) by this second polarization beam splitter 1006.
In the color combining system, the polarization direction of the G light reflected by the first polarization beam splitter 1005 is rotated 90° by the first reflective liquid crystal display element 1002G, the G light is transmitted through the first polarization beam splitter 1005, its polarization direction is again rotated 90° by a ½ wave plate 1012, is reflected by the third polarization beam splitter 1007, and reaches the projection optical system 1003.
Moreover, the polarization direction of the R light is rotated 90° and the R light is reflected by the second reflective liquid crystal display element 1002R, and is transmitted through the second polarization beam splitter 1006. The polarization direction of the B light is rotated 90° and the B light is reflected by the third reflective liquid crystal display element 1002B, and is reflected by the second polarization beam splitter 1006. Thus, the light of the two colors red and blue is combined into one light flux.
Here, the polarization direction of the B light is rotated 90° by a second color-selective wave plate 1009 arranged between the second polarization beam splitter 1006 and the third polarization beam splitter 1007, so that the B light becomes P-polarized light, like the R light. Therefore, as the R and B light is transmitted through the third polarization beam splitter 1007, the light of the three colors is combined and reaches the projection optical system 1003.
However, in the color combining system of the image projection apparatus in this conventional example, polarization beam splitters are used to combine the light of the first color, the light of the second color and the light of the third color, so that there is the problem of a loss of light and a mismatching of the color balance when the transmittance for P-polarized light decreases due to the incidence angle characteristics of the polarization splitting surfaces (films) provided on the polarization beam splitters.
FIG. 13 shows a graph of the transmittance of the polarization splitting surface for P-polarized light. The polarization splitting surface achieves a transmittance that is close to 100% when the P-polarized light that is transmitted through the polarization splitting surface satisfies the Brewster angle, so that when the incidence angle fluctuates, the incidence angle at the polarization splitting surface will deviate from the Brewster angle. Thus, the transmittance is lowered considerably, so that there is the problem that an angular fluctuation as shown in FIG. 13 occurs.
Furthermore, there are also conventional examples, in which a dichroic prism is used instead of a polarization beam splitter 1007. However in this case, the half-value wavelength separating the wavelengths into transmitted and reflected wavelengths shifts due to the incidence angle characteristics, so that the spectral characteristics tend to change. Thus, there is the problem of a loss of light and a mismatching of the color balance.
FIG. 14 illustrates the transmittance of a dichroic film. The dichroic film is made by arranging layers of different refractive index in an alternating manner. Dichroic characteristics by which a predetermined wavelength region is transmitted while other wavelength regions are reflected are attained by alternatingly forming a transmitting (wavelength) band in which light is transmitted and a reflecting (wavelength) band in which light is reflected, with the same equivalent refractive index.
In this case, the following relation is established between the central wavelength λ0 of the reflection band and the refractive index n and the film thickness d of the film material formed in alternating layers, as shown in FIG. 15, and the light ray angle θ with respect to the refraction surface:λ0=4×n1×d1×cos(θ1)=4×n2×d2×cos(θ2)Here, n1×d1×cos(θ1) and n2×d2×cos(θ2) are the equivalent film thicknesses.
From this relation, it can be seen that when the incidence angle of the light on the dichroic film changes and the light ray angle θ on the refraction surface changes, then the center wavelength of the refractive band changes too, so that the dichroic characteristics shift in the wavelength direction, and angular fluctuations as shown in FIG. 14 are the result.